1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with varying the musical waveshapes by deleting selected portions of a waveshape by means of a data mask.
2. Description of the Prior Art
Electronic musical tone generating systems embody a variety of techniques for creating a number of differentiable tones which can be selected by the musician to provide a tonal color setting appropriate to a musical performance. To this end, tone generators are provided which can provide a plurality of different tones that can be selected either singly or in combination in response to the actuation of a number of tone switches. Tone switches are often called stops as a remembrance of the controls used to control ranks of wind-blown organ pipes.
Tone synthesizers provide an important additional dimension to musical tone generation by creating tones in which the harmonic content is varied as a function of time. A simple tone having a waveshape that is repeated cyclically and endlessly quite rapidly fatigues a listener. This fatigue phenomena is a negative attribute of most simple tone generating schemes. Even a wind-blown organ pipe has a tone variant spectra produced by air turbulence and a constant and random acoustic phase interference in the auditorium containing the organ pipes.
Both in the interest of circuit implementation economy as well as a means to obtain special tonal effects, musical tone generators have been employed which use various forms of waveshape distortion to obtain several types of tone colors for each individual tone controlled by an associated tone switch. An example of such a waveshape distortion system is disclosed in U.S. Pat. No. 4,300,432 entitled "Polyphonic Tone Synthesizer With Loudness Spectral Variation." A combination of waveshape distortion and a sliding formant filter is disclosed in U.S. Pat. No. 4,300,434 entitled "Apparatus For Tone Generation With Combined Loudness And Formant Spectral Variation."
A system for distorting a basic musical waveshape stored in a digital data memory is described in U.S. Pat. No. 4,183,275 entitled "Electronic Musical Instrument." The distortion is obtained by changing the memory advance read out rate at preselected memory address points. The result is equivalent to a frequency modulation.
A common technique for distorting a musical waveshape is to use some form of signal modulation. Several musical instruments have been designed which employ frequency modulation subsystems to produce time variant distortions of a simple sinusoid signal at the musical tones fundamental frequency. Phase modulation subsystems produce exactly equivalent distortions as those achieved with frequency modulation.
Amplitude signal modulation has also been utilized in musical tone generators to modify the spectra of an input signal. Ring modulators, or balanced modulators, are almost a standard feature for tone generators of the synthesizer genre.
Most of the commonly implemented waveshape distortion techniques require the use of a data multiplier. This is a relatively expensive item for a digital tone generation system. Even with the current trend toward the use of low cost microelectronic structures, a binary data multiplier is still a comparatively expensive and low speed device.